FET blend

[flateric]

So I was looking for a simple blender to add to one of my tubescreamer overdrive pedals for bass guitar use, to let you mix a bit of the dry signal with the overdriven one - a bit of bass reinforcement, and I found a couple of Mark Hammer and Gausmarkoff circuits using 2 x TL072 op ams and loads of components. No room to add that to an already overstuffed enclosure. (Google B Blender circuit)
Then I found a really simple FET-based blender - perfect - mapped out the design to veroboard and ready to solder.
But why do you need a FET circuit at all for a blender to add in to an existing pedal? Why not just use a pot to blend wet'n'dry together?
Can anyone explain that to me?

[JohnH]

Hi Eric - once you mix the two signals using one or more pots, the output impedance will probably be higher than that directly from a single active output. This may not be a problem to go into a high impedance input like a guitar amp, but it can be a cleaner more robust output with an output buffer, which is what the FET may be doing.

What is your circuit?
This is my version:
Blender module

John

[flateric]

its very similar to your blender John, my photobucket acounts gone a bit funny or i'd post a pic, just a couple of slight value differences but working as yours thru a fet and couple of caps, 2 x 2m2 resistors and a 4k7, thats about it.

[sumgai]

♭eric,

There's more to it.

In essence, you aren't just asking for a mixer to blend two signals, you're asking to mix the same signal, albeit one is "clean" and the other is umm, err... modified! Yeah, that's it, modified.

That said, it doesn't matter if you use an active component for each incoming signal, or you use only a resistor (part of a potentiometer) - you still run the risk of the two signals being out of phase. Check carefully to try and make sure the two signals arrive at the blender in phase, compared to each other. I'm sure you can dope out what'll happen to your Mojo^™, should things be all cattywhampus in this regard.

Beyond that, unless you have a wild impedance mismatch, then an active component is not all the necessary, when considered in the context of other active circuitry. By itself, as merely a blending circuit for output to Gawd-knows-where, then yeah, that FET would be nice, and perhaps a life-saver.

HTH




sumgai